The present patent application relates to magnetic resonance imaging apparatus and methods for using such apparatus in surgical procedures.
In magnetic resonance imaging, an object to be imaged as, for example, a body of a human subject is exposed to a strong, substantially constant static magnetic field. The static magnetic field causes the spin vectors of certain atomic nuclei within the body to randomly rotate or “precess” around an axis parallel to the direction of the static magnetic field. Radio frequency excitation energy is applied to the body, and this energy causes the nuclei to “precess” in phase and in an excited state. As the precessing atomic nuclei relax, weak radio frequency signals are emitted; such radio frequency signals are referred to herein as magnetic resonance signals.
Different tissues produce different signal characteristics. Furthermore, relaxation times are a dominant factor in determining signal strength. In addition, tissues having a high density of certain nuclei will produce stronger signals than tissues with a low density of such nuclei. Relatively small gradients in the magnetic field are superimposed on the static magnetic field at various times during the process so that magnetic resonance signals from different portions of the patient's body differ in phase and/or frequency. If the process is repeated numerous times using different combinations of gradients, the signals from the various repetitions together provide enough information to form a map of signal characteristics versus location within the body. Such a map can be reconstructed by conventional techniques well known in the magnetic resonance imaging art, and can be displayed as a pictorial image of the tissues as known in the art.
The magnetic resonance imaging technique offers numerous advantages over other imaging techniques. MRI does not expose either the patient or medical personnel to X-rays and offers important safety advantages. Also, magnetic resonance imaging can obtain images of soft tissues and other features within the body which are not readily visualized using other imaging techniques. Accordingly, magnetic resonance imaging has been widely adopted in the medical and allied arts.
Many conventional magnetic resonance imaging instruments require that a patient lie on a horizontal bed that is then advanced into a tubular bore within a super-conducting solenoidal magnet used to generate the static magnetic field. These units force the patient to undergo an intensely claustrophobic experience while being imaged. Other forms of magnetic resonance imaging apparatus, commonly referred to as “open MRI apparatus,” were developed to provide a less claustrophobic experience to the patient and greater access to the patient by medical personnel during the imaging procedure. However, even in this improved apparatus, the patient was still positioned inside the apparatus, and medical personnel attending to the patient would reach into the apparatus from outside, so that components of the apparatus still obstructed access to some extent.
As described in U.S. Pat. Nos. 6,335,623 and 6,541,973, which are assigned to the assignee of the present application, the disclosures of which are hereby incorporated by reference herein, this problem can be solved completely by providing space within the apparatus itself to accommodate medical personnel in addition to the patient. Thus, as shown in certain embodiments disclosed in the '973 and '623 patents, the magnet may include a ferromagnetic frame incorporating a floor, a ceiling and a pair of side walls extending between the floor and the ceiling, a lower ferromagnetic pole structure projecting upwardly from the floor and an upper ferromagnetic pole structure projecting downwardly from the ceiling. The projecting pole structures define a patient-receiving space between them. The magnet also includes flux generating elements such as resistive or superconducting coils or permanent magnets arranged to direct flux through the frame so that the flux passes through the patient-receiving space between the pole structures and returns through the side walls, floor and ceiling. The space between the side walls may be of essentially any size, but is desirably sufficient so that medical personnel can enter into the space along with the patient. In effect, the frame forms a room with a pole structure projecting down from the ceiling and another pole structure projecting up from the floor. The medical personnel inside the room have essentially unobstructed access to the patient from any side. It is, thus, quite practical to perform surgery or other medical procedure on a patient while the patient is in the patient-receiving space of the MRI apparatus. The room defined by the magnet frame may be equipped with features normally found in operating rooms, so that the magnet effectively becomes an MRI-capable operating room. Thus, surgery or other procedures can be performed under MRI guidance.
As shown in detail in the '973 patent, a patient positioning device may include a chassis having a pair of vertically extending end portions or leg portions and a bridge portion extending between these leg portions. The end portions of the chassis are spaced apart by a distance greater than the dimension of the lower pole structure. A bed is movably mounted to the chassis so that the bed can move and pivot in various directions relative to the chassis. The chassis is provided with wheels so that the patient can be positioned in the patient-receiving space of the magnet by placing the patient on the bed and wheeling the chassis into position, with the end portions of chassis disposed on opposite sides of the lower pole structure and with the bridge portion of the chassis spanning across the lower pole structure, so that the bridge portion of the chassis and the bed lie within the patient-receiving space. The patient can then be repositioned in various ways as by turning the bed about a vertical axis, tilting the bed about a horizontal axis or sliding the bed relative to the chassis. These arrangements provide extraordinary versatility in imaging of the patient and in positioning the patient for medical procedures. However, still further improvement would be desirable. For example, the MRI magnet typically is equipped with a false floor covering the ferromagnetic floor. The wheels of the chassis rest on the false floor. Any vibration or movement of the false floor will result in corresponding movement of the patient relative to the magnet. Further, the frame of the magnet must be designed to accommodate the full range of positions and orientations without comprising the susceptibility of the magnet to other sources of vibrations. In addition, the patient positioning device must incorporate mechanical features such as bearings and slides to allow movement of the bed relative to the chassis. It is difficult to accommodate bearings and slides of sufficient strength to allow for all of the desired ranges of movement while still providing a firm, secure support.
The present invention addresses the foregoing needs.